DOCSLIB.ORG

A Precise Determination of Top Quark Electro-Weak Couplings at the ILC

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
A Precise Determination of Top Quark Electro-Weak Couplings at the ILC IFIC/13-06 LAL 13-265 A precise determination of top quark electro-weak couplingsp at the ILC operating at s = 500 GeV M.S. Amjad1, M. Boronat2, T. Frisson1, I. Garc´ıaGarc´ıa2, R. P¨oschl∗1, E. Ros2, F. Richard1,2, J. Rou¨en´e1, P. Ruiz Femenia2 and M. Vos2 1 LAL, CNRS/IN2P3, Universit´eParis Sud, F-91898 Orsay CEDEX, FRANCE 2IFIC, Universitat de Valencia CSIC, c/ Catedr´aticoJos´eBeltr´an,2 46980 Paterna, SPAIN September 10, 2018 Abstract Top quark production in the process e+e− ! tt at a future linear electron positron collider with polarised beams is a powerful tool to determine indirectly the scale of new physics. The presented study, based on a detailed simulation p of the ILD detector concept, assumes a centre-of-mass energy of s = 500 GeV and a luminosity of L = 500 fb−1 equality shared between the incoming beam polarisations of Pe−;+ = ±0:8; ∓0:3. Events are selected in which the top pair decays semi-leptonically. The study comprises the cross sections, the forward- backward asymmetry and the slope of the helicity angle asymmetry. The vector, axial vector and tensorial CP conserving couplings are separately determined for the photon and the Z0 component. The sensitivity to new physics would arXiv:1307.8102v1 [hep-ex] 30 Jul 2013 be dramatically improved with respect to what is expected from LHC for elec- troweak couplings. 1 Introduction The main goal of current and future machines at the energy frontier is to under- stand the nature of electro-weak symmetry breaking. This symmetry breaking can be ∗Corresponding author: [email protected] 1 generated by the existence of a new strong sector, inspired by QCD, that may man- ifest itself at energies of around 1 TeV. In all realisations of the new strong sector, as for example Randall-Sundrum models [1] or compositeness models [2], Standard Model fields would couple to the new sector with a strength that is proportional to their mass. For this and other reasons, the t quark is expected to be a window to any new physics at the TeV energy scale. New physics will modify the electro-weak ttX vertex described in the Standard Model by Vector and Axial vector couplings V and A to the vector bosons X = γ; Z0. Generally speaking, an e+e− linear collider (LC) can measure t quark electro- weak couplings at the % level. In contrast to the situation at hadron colliders, the leading-order pair production process e+e− ! tt goes directly through the ttZ0 and ttγ vertices. There is no concurrent QCD production of t quark pairs, which increases greatly the potential for a clean measurement. In the literature there a various ways to describe the current at the ttX vertex. Ref. [3] uses: ttX 2 X 2 X 2 (q − q)µ X 2 X 2 Γµ (k ; q; q) = ie γµ Fe1V (k ) + γ5Fe1A(k ) + Fe2V (k ) + γ5Fe2A(k ) : 2mt (1) with k2 being the four momentum of the exchanged boson and q and q the four vectors of the t and t quark. Further γµ with µ = 0; ::; 3 are the Dirac matrices describing vector currents and γ5 = iγ0γ1γ2γ3 is the Dirac matrix allowing to introduce an axial vector current into the theory Applying the Gordon identity to the vector and axial vector currents in Eq.1 the parametrisation of the ttX vertex can be written as: ttX 2 X 2 X 2 σµν µ X 2 X 2 Γµ (k ; q; q) = −ie γµ F1V (k ) + γ5F1A(k ) + (q + q) iF2V (k ) + γ5F2A(k ) ; 2mt (2) i X X with σµν = 2 (γµγν − γνγµ). The couplings or form factors Fei and Fi appearing in Eqs.1 and2 are related via X X X X X X X X X Fe1V = − F1V + F2V ; Fe2V = F2V ; Fe1A = −F1A ; Fe2A = −iF2A : (3) Within the Standard Model the Fi have the following values: γ;SM 2 γ;SM Z;SM 1 8 2 Z;SM 1 F1V = − ;F1A = 0;F1V = − 1 − sw ;F1A = ; (4) 3 4swcw 3 4swcw with sw and cw being the sine and the cosine of the Weinberg angle θW . All the expressions above are given at Born level. Throughout the article no γ attempt will be made to go beyond that level. The coupling F2V is related via 2 γ F2V = Qt(g − 2)=2 to the anomalous magnetic moment (g − 2) with Qt being the electrical charge of the t quark. The coupling F2A is related to the dipole moment d = (e=2mt)F2A(0) that violates the combined Charge and Parity symmetry CP . Due to the γ=Z0 interference the t pair production is sensitive to the sign of the individual form factors. This is in contrast to couplings of t quarks to γ and Z0 individually where the form factors enter only quadratically. The authors of [4] use LEP data to Z set indirect constraints on the electric form factors F1V;A. These limits are sufficient Z Z γ;Z to exclude an ambiguity for F1A but not for F1V . In [4] also rough estimates for jF2V j are given. Today, the most advanced proposal for a linear collider is the International Linear Collider, ILC [5], which can operate at centre-of-mass energies between the Z0 pole to 1 TeV. The ILC provides an ideal environment to measure thesep couplings. The tt pairs would be copiously produced, several 100,000 events at s = 500 GeV for an integrated luminosity of 500 fb−1. It is possible to almost entirely eliminate the background from other Standard Model processes. The ILC will allow for polarised electron and positron beams. With the use of polarised beams, t and t quarks oriented toward different angular regions in the detector are enriched in left-handed or right- handed t quark helicity [6]. This means that the experiments can independently access the couplings of left- and right-handed chiral parts of the t quark wave function to the Z0 boson and the photon. In principle, the measurement of the cross section t and forward-backward asymmetry AFB for two different polarisation settings allows extracting both, the photon and Z0 couplings of the t quark for each helicity state. This study introduces the angle of the decay lepton in semi-leptonic decays of the tt in the rest frame of the t quark. This angle will be called the helicity angle. The slope of the resulting angular distribution is a measure for the fraction of t quarks in left-handed helicity state, tL and right-handed helicity state, tR, in a given sample. There are therefore six independent observables • The cross section; t • The forward backward asymmetry AFB; • The slope of the distribution of the helicity angle; for two beam polarisations. For the extraction of the six CP conserving form factors 0 defined for the Z and the photon, F1V ;F1A and F2V , the following observations have to be taken into account: Close to the tt threshold the observables depend always on the sum F1V + F2V . Therefore a full disentangling of the form factors will be imprecise for energies below about 1 TeV. Hence, in the present study either the four form factors F1V;A are varied simultaneously, while the two F2V are kept at their 3 Standard Model values or vice versa. Throughout this article the CP violating form X factors F2A will be kept at their Standard Model values. This article is organised as follows. After this introduction the relations between the observables and the form factors are outlined before the experimental environment and the used data samples will be introduced. After that the selection of semi- leptonic decays of the tt pair will be presented and the selection efficiencies will be t given. The determination of AFB will be followed by the extraction of the slope of the distribution of the helicity angle. Potential systematic effects from experiment and theory uncertainties will be outlined. Finally the six CP conserving form factors will be extracted as explained above. This study goes therefore beyond earlier studies published in [7,8], 2 Top quark production at the ILC The dominant source of top quark production at a 500 GeV e+e− collider is electro- weak production. The Born-level diagram is presented in Figure 1(a). The decay of the top quarks proceeds predominantly through t ! W ±b. The subsequent decays of the W ± bosons to a charged lepton and a neutrino or a quark- anti-quark pair lead to a six-fermion final state. The study presented in this article focuses on the 'lepton+jets' final state l±νbbq0q. (a) tt pair production (b) Single top production Figure 1: Four diagrams that contribute to the e+e− ! lνbbq0q production: (a) Born-level tt pair production, (b) box diagram higher-order contribution to the same process (c) single top production (d) triple gauge boson production. Several other Standard Model processes give rise to the same final state. The most important source is single-top production through the process e+e− ! WW ∗ ! 4 W tb ! l±νbbq0q. One of the diagrams contributing to this process is presented in Figure 1(b). Another relevant source is the Z0WW production. Due to the coupling of initial state electrons or positrons to to W bosons both sources contribute nearly exclusively in a configuration with left handed polarised electron beams and right handed polarised positron beams. In that case single-top and Z0WW production would yield a production rate of order 10% and 5%, respectively, of that of the pair production diagram of Fig- ure 1(a).
Load more

Recommended publications
  • First Determination of the Electric Charge of the Top Quark
    First Determination of the Electric Charge of the Top Quark PER HANSSON arXiv:hep-ex/0702004v1 1 Feb 2007 Licentiate Thesis Stockholm, Sweden 2006 Licentiate Thesis First Determination of the Electric Charge of the Top Quark Per Hansson Particle and Astroparticle Physics, Department of Physics Royal Institute of Technology, SE-106 91 Stockholm, Sweden Stockholm, Sweden 2006 Cover illustration: View of a top quark pair event with an electron and four jets in the final state. Image by DØ Collaboration. Akademisk avhandling som med tillst˚and av Kungliga Tekniska H¨ogskolan i Stock- holm framl¨agges till offentlig granskning f¨or avl¨aggande av filosofie licentiatexamen fredagen den 24 november 2006 14.00 i sal FB54, AlbaNova Universitets Center, KTH Partikel- och Astropartikelfysik, Roslagstullsbacken 21, Stockholm. Avhandlingen f¨orsvaras p˚aengelska. ISBN 91-7178-493-4 TRITA-FYS 2006:69 ISSN 0280-316X ISRN KTH/FYS/--06:69--SE c Per Hansson, Oct 2006 Printed by Universitetsservice US AB 2006 Abstract In this thesis, the first determination of the electric charge of the top quark is presented using 370 pb−1 of data recorded by the DØ detector at the Fermilab Tevatron accelerator. tt¯ events are selected with one isolated electron or muon and at least four jets out of which two are b-tagged by reconstruction of a secondary decay vertex (SVT). The method is based on the discrimination between b- and ¯b-quark jets using a jet charge algorithm applied to SVT-tagged jets. A method to calibrate the jet charge algorithm with data is developed. A constrained kinematic fit is performed to associate the W bosons to the correct b-quark jets in the event and extract the top quark electric charge.
    [Show full text]
  • Searching for a Heavy Partner to the Top Quark
    SEARCHING FOR A HEAVY PARTNER TO THE TOP QUARK JOSEPH VAN DER LIST 5e Abstract. We present a search for a heavy partner to the top quark with charge 3 , where e is the electron charge. We analyze data from Run 2 of the Large Hadron Collider at a center of mass energy of 13 TeV. This data has been previously investigated without tagging boosted top quark (top tagging) jets, with a data set corresponding to 2.2 fb−1. Here, we present the analysis at 2.3 fb−1 with top tagging. We observe no excesses above the standard model indicating detection of X5=3 , so we set lower limits on the mass of X5=3 . 1. Introduction 1.1. The Standard Model One of the greatest successes of 20th century physics was the classification of subatomic particles and forces into a framework now called the Standard Model of Particle Physics (or SM). Before the development of the SM, many particles had been discovered, but had not yet been codified into a complete framework. The Standard Model provided a unified theoretical framework which explained observed phenomena very well. Furthermore, it made many experimental predictions, such as the existence of the Higgs boson, and the confirmation of many of these has made the SM one of the most well-supported theories developed in the last century. Figure 1. A table showing the particles in the standard model of particle physics. [7] Broadly, the SM organizes subatomic particles into 3 major categories: quarks, leptons, and gauge bosons. Quarks are spin-½ particles which make up most of the mass of visible matter in the universe; nucleons (protons and neutrons) are composed of quarks.
    [Show full text]
  • Three Lectures on Meson Mixing and CKM Phenomenology
    Three Lectures on Meson Mixing and CKM phenomenology Ulrich Nierste Institut f¨ur Theoretische Teilchenphysik Universit¨at Karlsruhe Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany I give an introduction to the theory of meson-antimeson mixing, aiming at students who plan to work at a flavour physics experiment or intend to do associated theoretical studies. I derive the formulae for the time evolution of a neutral meson system and show how the mass and width differences among the neutral meson eigenstates and the CP phase in mixing are calculated in the Standard Model. Special emphasis is laid on CP violation, which is covered in detail for K−K mixing, Bd−Bd mixing and Bs−Bs mixing. I explain the constraints on the apex (ρ, η) of the unitarity triangle implied by ǫK ,∆MBd ,∆MBd /∆MBs and various mixing-induced CP asymmetries such as aCP(Bd → J/ψKshort)(t). The impact of a future measurement of CP violation in flavour-specific Bd decays is also shown. 1 First lecture: A big-brush picture 1.1 Mesons, quarks and box diagrams The neutral K, D, Bd and Bs mesons are the only hadrons which mix with their antiparticles. These meson states are flavour eigenstates and the corresponding antimesons K, D, Bd and Bs have opposite flavour quantum numbers: K sd, D cu, B bd, B bs, ∼ ∼ d ∼ s ∼ K sd, D cu, B bd, B bs, (1) ∼ ∼ d ∼ s ∼ Here for example “Bs bs” means that the Bs meson has the same flavour quantum numbers as the quark pair (b,s), i.e.∼ the beauty and strangeness quantum numbers are B = 1 and S = 1, respectively.
    [Show full text]
  • Analytic Solutions for Neutrino Momenta in Decay of Top Quarks
    Analytic solutions for neutrino momenta in decay of top quarks Burton A. Betchart∗, Regina Demina, Amnon Harel Department of Physics and Astronomy, University of Rochester, Rochester, NY, United States of America Abstract We employ a geometric approach to analytically solving equations of constraint on the decay of top quarks involving leptons. The neutrino momentum is found as a function of the 4-vectors of the associated bottom quark and charged lepton, the masses of the top quark and W boson, and a single parameter, which constrains it to an ellipse. We show how the measured imbalance of momenta in the event reduces the solutions for neutrino momenta to a discrete set, in the cases of one or two top quarks decaying to leptons. The algorithms can be implemented concisely with common linear algebra routines. Keywords: top, neutrino, reconstruction, analytic 1. Introduction decay of the intermediate W boson falls outside experimental acceptance. Top quark reconstruction from channels containing one or more leptons presents a challenge since the neutrinos are not directly observed. The sum of neutrino momenta can be in- 2. Derivation ferred from the total momentum imbalance, but this quantity The kinematics of top quark decay constrain the W boson frequently has the worst resolution of all constraints on top momentum vector to an ellipsoidal surface of revolution about quark decays. Reconstruction at hadron colliders faces fur- an axis coincident with bottom quark momentum. Simultane- ther difficulties, since the longitudinal momentum is uncon- ously, the kinematics of W boson decay constrain the W boson strained. In a common approach to the single neutrino final momentum vector to an ellipsoidal surface of revolution about state at hadron colliders (e.g.
    [Show full text]
  • Quarks and Their Discovery
    Quarks and Their Discovery Parashu Ram Poudel Department of Physics, PN Campus, Pokhara Email: [email protected] Introduction charge (e) of one proton. The different fl avors of Quarks are the smallest building blocks of matter. quarks have different charges. The up (u), charm They are the fundamental constituents of all the (c) and top (t) quarks have electric charge +2e/3 hadrons. They have fractional electronic charge. and the down (d), strange (s) and bottom (b) quarks Quarks never exist alone in nature. They are always have charge -e/3; -e is the charge of an electron. The found in combination with other quarks or antiquark masses of these quarks vary greatly, and of the six, in larger particle of matter. By studying these larger only the up and down quarks, which are by far the particles, scientists have determined the properties lightest, appear to play a direct role in normal matter. of quarks. Protons and neutrons, the particles that make up the nuclei of the atoms consist of quarks. There are four forces that act between the quarks. Without quarks there would be no atoms, and without They are strong force, electromagnetic force, atoms, matter would not exist as we know it. Quarks weak force and gravitational force. The quantum only form triplets called baryons such as proton and of strong force is gluon. Gluons bind quarks or neutron or doublets called mesons such as Kaons and quark and antiquark together to form hadrons. The pi mesons. Quarks exist in six varieties: up (u), down electromagnetic force has photon as quantum that (d), charm (c), strange (s), bottom (b), and top (t) couples the quarks charge.
    [Show full text]
  • Higgs Boson Production in Association with a Top Quark Pair at √ S = 13
    Higgs boson production in association with a top quark pair p at s = 13 TeV with the ATLAS detector Robert Wolff on behalf of the ATLAS Collaboration CPPM, Aix-Marseille Universit´eand CNRS/IN2P3, 163, avenue de Luminy - Case 902 - 13288 Marseille, France A search for the associated production of the Higgs boson with a top quark pair (ttH¯ ) is performed in multileptonic final states using a dataset corresponding to an integrated lumi- nosity of 36.1 fb1 of proton-proton collision data recorded by the ATLAS experiment at a p center-of-mass energy of s = 13 TeV at the Large Hadron Collider. Higgs boson decays to WW ∗, ττ and ZZ∗ are targeted. Seven final states, categorized by the number and flavour of charged-lepton candidates, are examined for the presence of the Standard Model Higgs boson with a mass close to 125 GeV and a pair of top quarks. An excess of events over the expected background from Standard Model processes is found with an observed significance of 4.1 standard deviations, compared to an expectation of 2.8 standard deviations. Furthermore, the combination of this result with other ttH¯ searches from the ATLAS experiment using the Higgs boson decay modes to b¯b, γγ and ZZ∗ ! 4` is briefly reported. 1 Top quark Yukawa coupling at the LHC The Higgs boson has been discovered by the ATLAS and CMS experiments at the Run 1 of the Large Hadron Collider (LHC) with a mass close to 125 GeV1;2. Its measured properties like spin and interactions are consistent with those predicted for a Higgs boson by the Standard Model (SM) of particle physics.
    [Show full text]
  • Quark Masses 1 66
    66. Quark masses 1 66. Quark Masses Updated August 2019 by A.V. Manohar (UC, San Diego), L.P. Lellouch (CNRS & Aix-Marseille U.), and R.M. Barnett (LBNL). 66.1. Introduction This note discusses some of the theoretical issues relevant for the determination of quark masses, which are fundamental parameters of the Standard Model of particle physics. Unlike the leptons, quarks are confined inside hadrons and are not observed as physical particles. Quark masses therefore cannot be measured directly, but must be determined indirectly through their influence on hadronic properties. Although one often speaks loosely of quark masses as one would of the mass of the electron or muon, any quantitative statement about the value of a quark mass must make careful reference to the particular theoretical framework that is used to define it. It is important to keep this scheme dependence in mind when using the quark mass values tabulated in the data listings. Historically, the first determinations of quark masses were performed using quark models. These are usually called constituent quark masses and are of order 350MeV for the u and d quarks. Constituent quark masses model the effects of dynamical chiral symmetry breaking discussed below, and are not directly related to the quark mass parameters mq of the QCD Lagrangian of Eq. (66.1). The resulting masses only make sense in the limited context of a particular quark model, and cannot be related to the quark mass parameters, mq, of the Standard Model. In order to discuss quark masses at a fundamental level, definitions based on quantum field theory must be used, and the purpose of this note is to discuss these definitions and the corresponding determinations of the values of the masses.
    [Show full text]
  • 11. the Top Quark and the Higgs Mechanism Particle and Nuclear Physics
    11. The Top Quark and the Higgs Mechanism Particle and Nuclear Physics Dr. Tina Potter Dr. Tina Potter 11. The Top Quark and the Higgs Mechanism 1 In this section... Focus on the most recent discoveries of fundamental particles The top quark { prediction & discovery The Higgs mechanism The Higgs discovery Dr. Tina Potter 11. The Top Quark and the Higgs Mechanism 2 Third Generation Quark Weak CC Decays 173 GeV t Cabibbo Allowed |Vtb|~1, log(mass) |Vcs|~|Vud|~0.975 Top quarks are special. Cabibbo Suppressed m(t) m(b)(> m(W )) |Vcd|~|Vus|~0.22 |V |~|V |~0.05 −25 cb ts τt ∼ 10 s ) decays b 4.8GeV before hadronisation Vtb ∼ 1 ) 1.3GeV c BR(t ! W + b)=100% s 95 MeV 2.3MeV u d 4.8 MeV Bottom quarks are also special. b quarks can only decay via the Cabbibo suppressed Wcb vertex. Vcb is very small { weak coupling! Interaction b-jet point ) τ(b) τ(u; c; d; s) b Jet initiated by b quarks look different to other jets. b quarks travel further from interaction point before decaying. b-jet traces back to a secondary vertex {\ b-tagging". Dr. Tina Potter 11. The Top Quark and the Higgs Mechanism 3 The Top Quark The Standard Model predicted the existence of the top quark 2 +3e u c t 1 −3e d s b which is required to explain a number of observations. d µ− Example: Non-observation of the decay W − 0 + − 0 + − −9 K ! µ µ B(K ! µ µ ) < 10 u=c=t νµ The top quark cancels the contributions W + from the u and c quarks.
    [Show full text]
  • ELEMENTARY PARTICLES in PHYSICS 1 Elementary Particles in Physics S
    ELEMENTARY PARTICLES IN PHYSICS 1 Elementary Particles in Physics S. Gasiorowicz and P. Langacker Elementary-particle physics deals with the fundamental constituents of mat- ter and their interactions. In the past several decades an enormous amount of experimental information has been accumulated, and many patterns and sys- tematic features have been observed. Highly successful mathematical theories of the electromagnetic, weak, and strong interactions have been devised and tested. These theories, which are collectively known as the standard model, are almost certainly the correct description of Nature, to first approximation, down to a distance scale 1/1000th the size of the atomic nucleus. There are also spec- ulative but encouraging developments in the attempt to unify these interactions into a simple underlying framework, and even to incorporate quantum gravity in a parameter-free “theory of everything.” In this article we shall attempt to highlight the ways in which information has been organized, and to sketch the outlines of the standard model and its possible extensions. Classification of Particles The particles that have been identified in high-energy experiments fall into dis- tinct classes. There are the leptons (see Electron, Leptons, Neutrino, Muonium), 1 all of which have spin 2 . They may be charged or neutral. The charged lep- tons have electromagnetic as well as weak interactions; the neutral ones only interact weakly. There are three well-defined lepton pairs, the electron (e−) and − the electron neutrino (νe), the muon (µ ) and the muon neutrino (νµ), and the (much heavier) charged lepton, the tau (τ), and its tau neutrino (ντ ). These particles all have antiparticles, in accordance with the predictions of relativistic quantum mechanics (see CPT Theorem).
    [Show full text]
  • The Quark Model and Deep Inelastic Scattering
    The quark model and deep inelastic scattering Contents 1 Introduction 2 1.1 Pions . 2 1.2 Baryon number conservation . 3 1.3 Delta baryons . 3 2 Linear Accelerators 4 3 Symmetries 5 3.1 Baryons . 5 3.2 Mesons . 6 3.3 Quark flow diagrams . 7 3.4 Strangeness . 8 3.5 Pseudoscalar octet . 9 3.6 Baryon octet . 9 4 Colour 10 5 Heavier quarks 13 6 Charmonium 14 7 Hadron decays 16 Appendices 18 .A Isospin x 18 .B Discovery of the Omega x 19 1 The quark model and deep inelastic scattering 1 Symmetry, patterns and substructure The proton and the neutron have rather similar masses. They are distinguished from 2 one another by at least their different electromagnetic interactions, since the proton mp = 938:3 MeV=c is charged, while the neutron is electrically neutral, but they have identical properties 2 mn = 939:6 MeV=c under the strong interaction. This provokes the question as to whether the proton and neutron might have some sort of common substructure. The substructure hypothesis can be investigated by searching for other similar patterns of multiplets of particles. There exists a zoo of other strongly-interacting particles. Exotic particles are ob- served coming from the upper atmosphere in cosmic rays. They can also be created in the labortatory, provided that we can create beams of sufficient energy. The Quark Model allows us to apply a classification to those many strongly interacting states, and to understand the constituents from which they are made. 1.1 Pions The lightest strongly interacting particles are the pions (π).
    [Show full text]
  • Measurement of the Top Quark Mass with Lepton+Jets Final States Using Pp Collisions at Sqrt(S)
    EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-EP-2018-063 2018/11/06 CMS-TOP-17-007 Measurement of the top quark massp with lepton+jets final states using pp collisions at s = 13 TeV The CMS Collaboration∗ Abstract The mass of the top quark is measured using a samplep of tt events collected by the CMS detector using proton-proton collisions at s = 13 TeV at the CERN LHC. Events are selected with one isolated muon or electron and at least four jets from data corresponding to an integrated luminosity of 35.9 fb−1. For each event the mass is reconstructed from a kinematic fit of the decay products to a tt hypothesis. Us- ing the ideogram method, the top quark mass is determined simultaneously with an overall jet energy scale factor (JSF), constrained by the mass of the W boson in qq0 decays. The measurement is calibrated on samples simulated at next-to-leading or- der matched to a leading-order parton shower. The top quark mass is found to be 172.25 ± 0.08 (stat+JSF) ± 0.62 (syst) GeV. The dependence of this result on the kine- matic properties of the event is studied and compared to predictions of different mod- els of tt production, and no indications of a bias in the measurements are observed. Published in the European Physical Journal C as doi:10.1140/epjc/s10052-018-6332-9. arXiv:1805.01428v2 [hep-ex] 5 Nov 2018 c 2018 CERN for the benefit of the CMS Collaboration. CC-BY-4.0 license ∗See Appendix A for the list of collaboration members 1 1 Introduction The top quark plays a key role in precision measurements of the standard model (SM) because of its large Yukawa coupling to the Higgs boson.
    [Show full text]
  • The Discovery of the Top Quark
    The discovery of the top quark Claudio Campagnari University of California, Santa Barbara, California 93106 Melissa Franklin Harvard University, Cambridge, Massachusetts 02138 Evidence for pair production of a new particle consistent with the standard-model top quark has been reported recently by groups studying proton-antiproton collisions at 1.8 TeV center-of-mass energy at the Fermi National Accelerator Laboratory. This paper both reviews the history of the search for the top quark in electron-positron and proton-antiproton collisions and reports on a number of precise electroweak measurements and the value of the top-quark mass that can be extracted from them. Within the context of the standard model, the authors review the theoretical predictions for top-quark production and the dominant backgrounds. They describe the collider and the detectors that were used to measure the pair-production process and the data from which the existence of the top quark is evinced. Finally, they suggest possible measurements that could be made in the future with more data, measurements that would confirm the nature of this particle, the details of its production in hadron collisions, and its decay properties. [S0034-6861(97)00101-3] CONTENTS 2. Z tt background 174 → 3. Drell-Yan background 174 4. WW background 175 I. Introduction 137 5. bb¯ and fake-lepton backgrounds 175 II. The Role of the Top Quark in the Standard Model 138 6. Results 175 III. Top Mass and Precision Electroweak Measurements 140 B. Lepton 1 jets + b tag 176 A. Measurements from neutral-current experiments 140 1. Selection of lepton 1 jets data before b 1.
    [Show full text]